Automatic transmissions for vehicles often include a plurality of spring assemblies that are adapted to apply, typically via a die-case piston member, a biasing force against various clutch and friction plate assemblies that control the engagement and disengagement of the various gears in the automatic transmission. A typical clutch assembly is comprised of a series of discs called clutch plates which have floating between them an additional series of plates called friction plates. Prior to the engagement of the plates by the piston, the clutch plates and the friction plates act as separate components, each moving independently with respect to the other component. As the clutch plates are compressed by the piston into engagement with the friction plates, there is enough force generated to cause the clutch plates and friction plates to adhere to each other. Once this "squeezing" occurs, the clutch plates and friction plates move as one, and so do their respective components. As the piston is moving to engage the clutch and friction plates, it is also compressing a spring assembly. The spring assembly biases the clutch and friction plate assemblies into their disengaged position. These prior art spring assemblies generally comprise an annular plate having a multiplicity of circumferentially spaced, parallel-oriented compression springs mounted thereto. Alternately, the spring assembly may include a second annular plate secured to the opposite free ends of the compression springs as well. Spring assemblies of the above-described type are disclosed in U.S. Pat. Nos. 3,782,708 and 5,306,086.
With the advent of automatic transmissions having four, five and even six forward gears, coupled with the need to package these transmissions into smaller overall volumes, there is a desire among the automobile manufacturers to reduce the size of the spring assemblies while at the same time increasing the axial force of the spring assemblies. The increase in the axial force is further required due to the increased amount of power being generated by today's engine and the requirement that the transmissions transmit this power to the remainder of the drive train of the automobile.
The prior art spring assemblies which consist of multiple round wire compression coil springs supported by one or possibly two annular retainers have experienced numerous constraints as the newer and smaller automatic transmissions are being designed and produced. Prior art spring assemblies do not make efficient enough use of the space available. Examples of non-utilized space include the center area of each individual coil spring and the areas around and between the individual coil springs. In some applications, in order to achieve the desired specifications, either the physical size or the required quantity of the individual coil springs renders the prior art spring assemblies ineffective. In order to extend the use of the prior art spring assemblies when space limitations are crucial, individual coil springs with a "space-filling" shape are combined with a minimum of inactive spring wire (i.e., using plain or partially closed ground ends). These shaped springs are then fabricated with a high tensile strength material in order to meet the required specifications.
In addition to the problems associated with meeting today's performance specification, the prior art spring assemblies all suffer from the problems associated with a complex multiple piece assembly and the reliability issues involved in its manufacture, installation and service.
A major concern in the clutch release spring industry is a phenomenon called shudder. There are many contributing factors which cause shudder, one of the larger known causes being the uneven pressures which are placed on the piston as the piston is being compressed. This uneven pressure causes the piston to tilt or cock within the clutch assembly and apply forces to one area of the clutch plates before the other. By applying uneven forces in this manner, true engagement of the clutch plates and friction plates does not occur and slippage of the assemblies is prevalent. Not only is slippage undesirable for the noise, vibration and harshness of the vehicle, it can also lead to early wear and possibly failure of the clutch assemblies.
The prior art spring assemblies have addressed these problems by incorporating the annular retainers to evenly absorb and distribute loads and by moving the multiple round wire compression springs radially outward as far as possible to retain a low, safe slenderness ratio of the spring assembly. (It's overall height compared to its overall mean diameter).
Accordingly, it is the primary objective of the present invention to provide a single piece spring which replaces the prior art spring assemblies while providing for an increase in the performance of the spring within the transmission assembly. In addition, it is also an object of the present invention to provide a single piece spring which satisfies the increasing performance specification in a reliable and cost-effective manner.